1. Field of the Invention
The present invention relates to a device for mixing and delivering orthopaedic bone cement or the like.
2. Description of the Prior Art
Orthopaedic bone cement is used throughout the world to secure hip, knee and other metallic prostheses in an appropriate anatomical position. The bone cement is produced by thoroughly mixing together two components, usually methylmethacrylate monomer liquid and polymethylmethacrylate powder. The mixing is usually carried out using a simple bowl and spatula. The surgeon then removes the required amount of cement and manipulates it by hand before inserting it into a preformed cavity or applying it to a resected bony surface where the prosthesis is to be positioned. Cement may either be applied by hand or may be put into a syringe and applied thereby. However, this simple mixing method has two major drawbacks.
Firstly, free methylmethacrylate fumes are emitted from the mixture. It is desirable to remove these fumes, or prevent them from escaping into the atmosphere, since they have an unpleasant odour and may be harmful to operating room personnel. The fumes are known to cause nausea and giddiness and are generally objectionable, particularly to the nurses who actually carry out the mixing. Recently there has also been concern that long term exposure to these fumes results in a more serious health risk. Current employment law relating to occupational health dictates that medical staff must now be protected against the exposure to hazardous substances.
Secondly, a very high mixing efficiency is required to produce a homogenous cement material. During the mixing process air is naturally introduced into the mixture since air is inherently existent within the powder and also in and around the mixing vessel. Air bubbles are also produced by the xe2x80x9cboiling offxe2x80x9d of monomer which occurs during the mixing process. The introduction of air produces a weak cement and, since the joint must usually support a heavy load, it is important to reduce the amount of air in the mixture as much as possible in order to improve the mechanical strength of the cement material.
In order to eliminate as much air as possible from the mixture, mixing is now preferably carried out under vacuum. This considerably reduces the amount of air in the mixture. Mixing in a conventional bowl and spatula system can produce a product with a porosity value of approximately 20 to 25%. In a vacuum mix, the porosity is often reduced to levels below 5%.
Several devices for mixing the cement in a vacuum are presently available. Some of these are in the form of hand-held mixing bowls. The substances to be mixed are placed in the bowl to which a vacuum is applied. The substances are mixed by means of a rotating paddle extending into the bowl which is rotated manually by means of a handle extending through the lid of the bowl. In some applications, the use of such a mixing bowl, an example of which is disclosed in WO 93/10892, is favoured. Many surgeons prefer to xe2x80x9chand packxe2x80x9d the cement. Bowl mixing also tends to be preferred by nurses who are used to the convenience of mixing in such a vessel; a bowl is easy to use and it is important that the nurses feel confident since timing is very crucial and the mixture must be xe2x80x98right first timexe2x80x99. Many surgeons also tend to prefer bowl mixers because they can easily take samples of the cement from the bowl at any time to determine the progress of polymerisation as it is crucial that the mixture does not begin to set before it is applied.
However, in some applications it is preferable or necessary to apply the mixed cement to the bone by means of a syringe. Indeed some surgeons, particularly in Europe, prefer syringe-type application to xe2x80x9chand packingxe2x80x9d. If the cement is mixed in a bowl, it must then be transferred to a dispensing syringe which can be messy and time consuming and may expose the mixture to more air entrapment. This problem has been overcome by combining a mixing chamber with a syringe. For example, EP-A-0178658 discloses a device for mixing bone cement comprising a mixing container connected to a feed device. A vacuum source is connected to the feed device for mixing the substances under vacuum. This device has proved to be a very efficient mixing and transfer system and eliminates the need to transfer the mixed cement from the mixing bowl to a syringe.
However, a device such as disclosed in EP-A-0178658 may be inadequate in that xe2x80x98dead spotsxe2x80x99, i.e. areas where the components are not sufficiently mixed, occur, particularly at the outer edges of the mixing chamber.
Further, the mixing paddle of EP-A-0178658 is rotated by a rotary electric drive motor. This makes the device costly and space consuming and requires specialist and time-consuming installation. The device is not easily portable and its use is, therefore, not particularly flexible.
U.S. Pat. No. 4,758,096 also discloses a bone cement mixer in which the cement is mixed in the dispensing vessel. In this device, the mixing is effected manually by means of a xe2x80x9cmasherxe2x80x9d plate-type agitator. The masher plate is attached to a shaft attached to a handle. The agitator is movable in the chamber both axially and rotatably to permit mixing of the cement by the user moving the handle vertically and rotatably. However, such a mixing operation is difficult and inefficient and does not result in thorough mixing of the cement, leaving areas of unmixed powder.
The present invention aims to overcome the above-mentioned problems.
According to one aspect of the invention, there is provided a bone cement mixing apparatus comprising a mixing chamber, a mixing element rotatably mounted in said mixing chamber, and drive means for causing rotation of said mixing element, wherein said drive means comprises a handle axially movable relative to said mixing chamber, and a gear mechanism between said handle and said mixing element such that said gear mechanism translates axial movement of said handle into rotation of said mixing element.
According to another aspect of the invention, there is provided a method of mixing bone cement in a mixing chamber, by a mixing element rotatably mounted in said chamber containing the material to be mixed; said method comprising axially moving a drive handle, relative to said chamber, wherein said handle is connected to said mixing element via a gear mechanism, such that axial movement of said handle is translated via said gear mechanism into rotation of said mixing element.
The preferred type of gear mechanism for use in the practice of the present invention is a xe2x80x9cbarley twistxe2x80x9d mechanism wherein a downwards stroke of the handle moves a threaded rod axially through a threaded gear housing in the form of a drive bush coupled to the mixing element imposing a rotational force in a first direction on the mixing element. On the upward stroke of the handle, the mixing element is caused to rotate in the opposite direction. Rotating the element alternately in different directions during mixing provides very thorough mixing. The xe2x80x9cbarley twistxe2x80x9d mechanism allows this with a simple push-pull action of the handle rather than the user having to rotate the handle backwards and forwards alternately which can be difficult and tiring, particularly when mixing cement of high viscosity.
Although the present invention may be applied to any bone cement mixing chamber, it is preferably used in combination with a dispensing syringe. Thus, the cylinder of the syringe forms the body of the mixing chamber and a plunger is slidably mounted at one end of the cylinder for causing ejection of the mixed cement.
As discussed above, the quality of the cement is greatly improved if it is mixed under vacuum and, therefore, in a preferred embodiment, the invention comprises means for creating a vacuum within the chamber.
For thorough mixing, the mixing element is preferably in the form of a paddle arrangement mounted on a shaft which in the preferred embodiment includes an axial bore through which the threaded rod of the barley twist gear mechanism can extend. The paddle or paddles extend radially from the shaft and at least one paddle may extend from the shaft to the wall of the mixing chamber so that as it rotates it wipes out the whole of the cement containing part of the interior of the chamber.
To avoid wasting any of the mixed cement, the mixing paddle should preferably be wiped clean before it is removed from the mixing chamber. A hygienic way of doing this, which avoids contamination of the cement is to provide a slot in the chamber through which the mixing paddle extends into the chamber. This slot is of a width substantially equal to the thickness of the mixing paddle so as to wipe any residual cement from the mixing paddle as it is withdrawn through the slot after mixing.
The slot may be formed as an integral part of the cylinder towards the end to which the handle is attached, in which case, after mixing, the cement would be ejected through this slot. Alternatively, the slot could be formed in a separate member, such as a cap or a plate, adapted to be inserted between the cylinder and the lid. The slotted member could then be removed after the mixing paddle has been withdrawn through it, before the cement is ejected.
In mixing bone cement, alternate layers of cement powder and monomer liquid are put into the mixing chamber. Initially the cement is very stiff and a high force is needed to start the mixing. There is then a surface reaction between the powder and liquid phases and once mixing has started the required mixing force drops sharply. The larger the interface area between the layers, the better the surface reaction. Thus, the cylinder should be as wide as possible to maximise the layer interface area whilst still being comfortable to handle.
In one embodiment, the mixing element, the handle and the gear mechanism are formed as an integral unit in a detachable lid assembly. The lid is adapted to be attached to one end of the cylinder by e.g. matching threaded portions. After mixing, the lid assembly including the mixing assembly comprising the mixing element, the handle and the gear mechanism can be removed from the cylinder, e.g. by unscrewing, and, in the preferred embodiment, can be replaced by a nozzle or the like. The nozzle cooperates with the plunger to eject the mixed cement out through the nozzle when the plunger is pushed into the cylinder. The plunger may be manually operated, e.g. using a hand gun arrangement or, alternatively, a gas powered pressure gun could be used.
In many applications, however, e.g. in hip replacement operations, more than one step is involved and, therefore, several batches of cement need to be mixed. To make the system more economical, the handle and the gear mechanism should preferably be re-usable, whereas the mixing paddle is only used once and then disposed of. Thus, for such applications the drive mechanism should be detachable from the mixing element after use.
According to a third aspect of the invention, there is provided a bone cement mixing apparatus comprising a mixing chamber, a mixing element rotatably mounted in said mixing chamber, and drive means including a handle for causing rotation of said mixing element, wherein said drive means is carried by a lid assembly of the chamber, and wherein the lid assembly and drive means are detachable from the chamber and from the mixing element thereby enabling reuse of the lid assembly and drive means with a different chamber and mixing element if desired.
Air tight seals are preferably provided between the lid assembly and the mixing element, between the lid assembly and the chamber and the chamber and the stand assembly. In this embodiment the drive bush of the preferred gear mechanism is preferably detachable from the paddle and may conveniently be push fitted into the top of the paddle shaft. A rib and groove locating arrangement is preferred.
When the contents of the chamber have been sufficiently mixed, the lid assembly and drive means can be removed, leaving the paddle inside the chamber. This indicates to the nurse that the next step is to remove the paddle and attach a nozzle for applying the cement. If the paddle is withdrawn through a slit to wipe it clean, it is easier to align the paddle with the slit once the lid assembly has been removed. This makes the device particularly user friendly.
The nozzle cooperates with the plunger to eject the mixed cement out through the nozzle when the plunger is pushed into the cylinder. The plunger may be manually operated, e.g. using a hand gun arrangement or, alternatively, a gas powered pressure gun could be used.
According to another aspect, the invention provides an orthopaedic bone cement mixing apparatus comprising a cylindrical syringe body defining a mixing chamber, a plunger slidably mounted at one end of the cylinder, a mixing member rotatably mounted in said chamber and drive means for causing rotation of said mixing member, wherein said mixing member includes a blade mounted on and radially extending from a rotatable shaft along the central axis of the cylinder such that rotation of said shaft causes said blade to rotate about the axis of the shaft within the interior of the cylinder, and wherein the blade extends from the shaft to the inner wall of the cylinder and is adapted and arranged so that as the blade rotates it wipes out the whole cement containing part of the interior of the cylinder.